The manufacture of cheese, yogurt, buttermilk, and similar dairy products requires the use of beneficial lactic acid-producing bacteria. In one procedure which has been in use for many years, the initial cultures are obtained from a culture laboratory, which may be a laboratory engaged in the business of selling cultures to the dairy industry or a laboratory maintained by the cheese manufacturer. These cultures are then used to prepare bulk starter cultures, which are then used to inoculate the milk in the cheese vats. Typical concentrations of such initial cultures have been about 2 .times. 10.sup.9 to 8 CFU (Colony Forming Units) per gram. The direct addition of such initial cultures to the milk in the cheese vats would result in an inefficient process, unless vary large volumes of such culture were used. Although the intermediate step of preparing the bulk starter culture ads to the expense of the operation of the cheese plant, it has been necessary in keeping the production cycle in the cheese vats to a minimum time.
In recent years, culture laboratories have begun to supply culture concentrates which can be used for direct inoculation of the milk in the cheese vats without the necessity of preparing a bulk starter. This process is referred to as the Direct Vat Set or DVS process. For DVS use, it is desirable to have the bacterial cultures as concentrated as possible. Centrifugation is used as the standard means of concentration. With the centrifugal apparatus commonly employed for this purpose, the aqueous liquid is discharged from the centrifuge and the bacteria collect on the bowl of the centrifuge from whence they may be scraped out or mechanically ejected during the operation of the centrifuge. Since the fermented culture contains residual nutrients some of which are in the form of undissolved solid particles, it is not always possible to make a clean separation of the bacterial cells from the medium and residual nutrients. Some cells are lost with the discharge liquid, and part of the residual nutrients are retained with the recovered cells. The theoretical objective, of course, is to recover 100% of the cells completely free of media constituents which, while not harmful, limit the obtainable cell concentration per unit volume. There has, therefore, been a search for means to improve the cleanness of the cell separation without sacrificing cell recovery particuarly when milk and/or whey constitute the growth media.
It has been found that somewhat higher cell concentrations can be produced when citrate ions are present in the media during the centrifugation. Sodium citrate is the reagent which has been used primarily for this purpose. However, the obtainable concentrations have still not been as great as desirable, and when sufficient citrate is present to promote the separation of the cells, it may have an adverse effect on the activity of the culture. Another procedure which has been used to some extent is the addition of a proteolytic enzyme to the fermented culture as described in U.S. Pat. No. 2,838,443. The enzyme treatment is of particular value in improving the cleanness of the cell separation when milk protein (casein) is present in the media.
With either citrate or enzyme, it has been difficult to regularly obtain with most mixed strain lactic streptococcus cultures cell concentrates containing as much as 50 .times. 10.sup.9 CFU per gram. However, concentrations of 100 .times. 10.sup.9 CFU per gram or higher would be desirable for DVS, especially if these can be obtained without reducing cell recovery.